The present invention relates to convergent-divergent nozzles used in gas turbine engines. In particular, the present invention relates to convergent-divergent nozzles with a floating area ratio.
Prior gas turbine engines have, in some configurations, included exit nozzles attached to the aft end of the engine. Exit nozzles are commonly employed to produce additional thrust for the engine by accelerating the working medium gas, for example air, leaving the aft end of the main engine, for example via the low pressure turbine, through the nozzle. Exit nozzles accelerate the air leaving the engine, and therefore produce useful thrust, by prescribing the nozzle area for particular exit pressures inside the nozzle. One such exit nozzle is the variable convergent-divergent nozzle. Prior variable convergent-divergent nozzles commonly include convergent-divergent flap sets arranged circumferentially about the main axis of the engine to form a substantially circular annular nozzle extending aft of, for example, the low pressure turbine. The convergent-divergent flap sets are commonly connected to an annular ring, sometimes referred to as a sync ring, which is in turn connected to an engine casing. The convergent flap in each of the convergent-divergent flap sets declines generally toward the main axis of the engine as the flap extends aftwardly. The divergent flap in each of the convergent-divergent flap sets may be pivotally connected to the convergent flap and inclines generally away from the main axis of the engine as the flap extends aftwardly. The circumferentially arranged convergent-divergent flap sets therefore form an annular nozzle whose cross-sectional area decreases from the forward end of the nozzle to a throat generally defined by the pivotal connection between the convergent and divergent flaps and then increases from the throat to the nozzle exit.
In order to operate efficiently, variable convergent-divergent nozzles are configured to position the convergent and divergent flaps, and thereby the annular shape of the entire nozzle, to optimize engine performance. The position of the convergent and divergent flaps, and thereby the annular shape of the nozzle is commonly represented by the ratio of the cross-sectional area of the nozzle at the exit (AE) divided by the cross-sectional area of the nozzle at the throat (AT), or AE/AT. The nozzle pressure ratio (NPR) is equal to the total pressure at the nozzle throat (PT) divided by the ambient pressure (PAmb), or NPR=PT/PAmb. Convergent-divergent nozzles function generally by designing AE/AT for critical flight conditions (NPR) in order to produce useful thrust by extracting as much energy as is practicable from the working medium gas flowing through the nozzle.
Prior variable convergent-divergent nozzles have used various means to vary the position of the convergent and divergent flaps for different engine conditions. For example, some prior convergent-divergent nozzles have mechanically prescribed the position of the convergent and divergent flaps through a kinematic mechanism driven by one or more actuators to tune AE/AT for specific engine conditions. Prior convergent-divergent nozzles have also employed kinematics that vary AE/AT with respect to AT to achieve improved performance at multiple engine operating conditions. This arrangement allows for a single valued AE/AT for all AT without the weight and complexity of independently controlling AE/AT. Other prior convergent-divergent nozzles have employed a toggling configuration triggered by the pressure inside the nozzle, which acts to position the divergent flaps for low and high AE respectively (low and high mode). Nozzles employing such a toggling configuration are considered to have two available values of AE for each AT. At low AT, which is typical of aircraft cruise and low values of NPR, a low value of AE/AT is desirable. At relatively high values of AT a higher value of AE/AT is desirable, which corresponds to conditions associated with aircraft acceleration. Thus the low mode (low AE/AT) condition corresponds to relatively low values of NPR and high mode (higher AE/AT) corresponds to relatively high values of NPR.
Prior variable convergent-divergent nozzles have several disadvantages with respect to AE/AT. In prior nozzles independently controlling AT and AE, one disadvantage is weight and complexity (design and control). For scheduled (single valued) AE/AT nozzles, one disadvantage is an inability to run optimally at low and high NPR (cruise and acceleration). More generally, prior nozzle designs have varied the nozzle geometry to optimally position the convergent and divergent flaps at low mode and high mode, but have failed to vary the convergent and divergent flaps position through an intermediate mode of engine operation between low and high modes. Therefore, prior nozzle configurations have failed to advantageously position the convergent and divergent flaps for a substantial number of NPR values encountered during engine operation, thereby causing sub-optimal engine performance at many of the NPR values encountered.